Make service_account_email public, read-only (#76)

diff --git a/google/oauth2/service_account.py b/google/oauth2/service_account.py
index 6159301..24c852b 100644
--- a/google/oauth2/service_account.py
+++ b/google/oauth2/service_account.py
@@ -232,6 +232,11 @@
             subject=self._service_account_email)
 
     @property
+    def service_account_email(self):
+        """The service account email."""
+        return self._service_account_email
+
+    @property
     def requires_scopes(self):
         """Checks if the credentials requires scopes.